Port test suite from nose to pytest.

This just eliminates all errors on the tests collection. Elimination of
failures is left to the next commit.

tests/test_managedblockchain/test_managedblockchain_proposalvotes.py

@@ -5,7 +5,7 @@ import os
 import boto3
 import sure  # noqa
 from freezegun import freeze_time
-from nose import SkipTest
+from unittest import SkipTest
 
 from moto import mock_managedblockchain, settings
 from . import helpers